An electric power steering device includes a motor 12 for giving a steering assist force to a steering system and a controller 15 for controlling a turning on state of the motor 12 in accordance with a steering torque of the steering system. The motor 12 and the controller 15 are respectively formed with separate units. Further, the motor 12 has a flange part 13 attached to an attaching member of a steering mechanism. The flange part 13 further includes a flange extension part 14 formed by projecting a part of the flange part in a direction of circumference. The controller 15 is attached to the flange extension part 14 to be mounted on an output shaft side B of the motor 12.